A device of this type is used, for example, in the espresso coffee machine named "coffee GAGGIA" of Messrs. BREVETTI GAGGIA S.p.A. in Robecco S/Naviglio (MI). This device is composed of a steam pipe connected to the steam heater of the espresso coffee machine, at the end of which pipe a nozzle with an orifice is provided.
To froth and heat milk, a user must hold the orifice of the nozzle at a defined distance from the surface of the milk so that as much air as possible is directed into the milk, which will then be enclosed as air bubbles in the milk. However, which distance is to be kept can only be determined roughly by several attempts.
If, for example, the orifice of the nozzle is immersed too deep into the milk, no air at all will be directed into the milk, and the steam only condenses to water in the milk and, further, disperses its heat to the milk. On the other hand, if the orifice of the nozzle is kept at too far a distance from the surface of the liquid, no milk froth will be produced in this case either, since the steam-and-air mixture is not blown into the milk sufficiently deep in order to emulsify with the milk. Thus, in practice, a user will move the surface of the milk up and down in respect of the nozzle in order to thereby arrive at the distance which is optimal for good froth results, at least every now and then, with the objective of obtaining a fairly good milk froth.
Further, DE-A1-35 38 041 discloses a device for the heating and emulsification of milk, wherein the steam pipe section and the subsequent nozzle are encompassed by a pipe of larger diameter, forming an annular chamber with these parts. The pipe projects beyond the orifice of the nozzle and tapers towards its end, where another orifice is provided which extends concentrically to the orifice of the nozzle and the diameter of which is substantially identical with, or slightly larger than, the diameter of the nozzle. Apertures to aspirate milk are provided at the pipe at the level of the nozzle's orifice. Far above the nozzle, another aperture is provided at the pipe which serves as an entry for the air.
This device works according to a completely different principle as compared to the device described before, since it can be immersed into the milk as deep as desired, however, at most until the position of the upper aperture. This is because the nozzle in this device is a nozzle according to the immersion principle. When hot steam exits from the orifice of the nozzle, vacuum develops in the annular chamber ensuring, on the one hand, that air is aspirated through the upper aperture, and milk is aspirated through the lower apertures and is then frothed by the entering air and, subsequently, exits from the opening provided at the tip of the pipe and is directed into the milk. Not only is this device complicated, in addition, the milk is not frothed as good as this is done by way of the single nozzle described hereinabove, the device's efficiency being limited.